Scour is considered one of the major causes of highway bridge failures in the United States. It is especially prevalent during floods and periods of rapid river flow activities. During floods, erosion of the foundation materials around and below the bridge piers causes structural instability. This process is dynamic, where erosion takes place near the peak flow rates, and deposition of sediments occur during the descending stages of the flood. If local scour is not identified in time, the structural integrity of the foundation progressively deteriorates and leads to severe damage and collapse of the bridge.
According to the data from the National Bridge Inventory (NBI), 484,546 highway bridges out of an inventory of 590,000 in the United States cross over waterways. Sixty percent of these bridges have been declared scour critical (Hunt and Price, 2003; Gee, 2003). The 1987 catastrophic collapse of the Schoharie Creek Bridge in New York State due to scour was one of the most severe bridge failures in the United States. Considering the consequences of scour damage, Federal Highway Administration (FHWA) issued a Technical Advisory in 1988 revising the National Bridge Inspection Standards (NBIS) to require evaluation of all bridges for susceptibility to damage resulting from scour. This issue is not only confined to the US boundaries. Local scour was identified as a high priority research need for infrastructure by the North American Euro Pacific Workshop for Sensing Issues in Civil Structural Health Monitoring (Ansari, 2004). Participants of this workshop comprised of government highway agency engineers as well as researchers from academia and industry from US and other countries.
Local scour is caused by the interference of bridge piers with the water flow and is characterized by the formation of scour holes resulting from clear-water scour or live-bed scour. Clear-water scour occurs when the bed materials upstream of the scour area is at rest. The maximum local scour depth is reached when the flow can no longer remove bed material from the scour area. Live-bed scour occurs when there is general sediment transport by the river.
A great amount of effort has been expended for the research and development of scour monitoring sensors and systems. Applicability of the existing methodologies however has been limited considering issues pertaining to the complexity and cost effectiveness, resolution, capability for providing repeated and reliable information, installation, and rigor in data retrieval and processing. The great amount of effort is illustrated by the wide array of methodologies that have been used in attempts to develop scour monitoring sensors. These methodologies include sonar (Mason et al., 1994; Hays et al., 1995), time domain reflectometry or TDR (Dowding et al., 1994; Yankielun et al., 1999), sliding collar (Lagasse et al., 1997; Richardson et al., 1994), radar (Gorin et al., 1989), piezoelectric (Lagasse et al., 1997), and the seismic transducer techniques (Zabilansky, 1996).
Radar and sonar based techniques have been successful in monitoring the scour depth after the flood event. However, their applicability for monitoring the scour event in real time has been limited and both techniques involve rigorous data processing and interpretation schemes. The information provided by battery operated devices including those based on neutral buoyancy of seismic transducers are crude and, in general, these devices have limited active lives. Buried mechanical devices such as magnetic collars are comparatively inexpensive. However, it is not possible to reset these devices for reuse and issues pertaining to binding and installations have hindered their usage. Techniques based on TDR either use sacrificial sensors that break off during scouring events or solely depend on the impedance mismatch and not practical for real applications involving various types of sedimentations. Attenuation and pulse dispersion errors due to length of electrical cables as well as probe length limitations are amongst other deficiencies of these systems. Sensors based on spatial positioning of PZT or fiber optic sensors (Bin et al., 2006) on a rod that can be driven into the sediment provide only incremental resolution. Moreover, these multi-sensor arrangements are expensive since they require sophisticated multi-channel data acquisition and interpretation techniques.
Therefore, none of the currently available scour monitoring techniques possesses the necessary attributes for widespread deployment in scour critical bridges. The desired device must be: accurate, simple in principle, easy to install and operate, simple to calibrate, cost effective and reliable. In addition, the sensor has to survive many floods and operate maintenance-free over a long period of time. These attributes provide the authorities with the necessary tools to make decisive actions for maintenance as well as securing the safety of the traveling public. The payoffs are significant in terms of tremendous cost savings for the federal and state highway agencies considering the current yearly levels of scour damage and bridge failure related expenditures.
Not withstanding the difficulty of determining scour, it can also be difficult to determine the subgrade modulus of the river bed, or the subgrade modulus for any soil or material for that matter, without unduly disturbing the material or affecting the soil or material once it has been placed in a desired location such as at a construction site.